Reference is made to my U.S. Pat. No. 3,658,420 issued Apr. 25, 1972, which describes an optical spatial filtering technique for detecting hole-type defects and excess spot defects in photomasks used in making micro-circuits. According to the patent, an approximate form factor intensity filter provides suppression of the regularly shaped mask features. Thus, for masks with features whose boundaries are along only the X-Y direction, the filter is a cross placed in the transform plane. With the rectangular features suppressed, primarily non-rectangular defect data passes so that spots as small as 0.1 mil are detected.
The present invention is directed to improvements over the methods of my earlier patent, as will become apparent as the description proceeds.